The largest fraction of the heat energy received by the atmosphere is derived from the condensation of water vapor, primarily from oceanic evaporation. The area of the ocean surface is almost three times that of land so heat flow changes at the air-water interface of the ocean have profound meteorological and climatological consequences. An understanding of the processes which occur at the air-water interface is fundamental to an understanding of atmospheric behavior.
The temperature at the interface between the ocean and the air is typically about 0.5 degrees Centigrade lower than the temperature immediately below the surface. This reduced temperature is caused by evaporative cooling at the surface of the ocean and is referred to as the sea-surface temperature (SST). The SST depends partly on physical processes occurring within the ocean itself, such as convective stirring and turbulent mixing, and partly on the influence of the overlying atmosphere. The SST exhibits complex patterns, with warm and cold areas in close juxtaposition, especially in regions where diverging and converging currents are found. These regions of strong horizontal temperature gradients are called oceanic fronts.
The SST is normally measured from surface vessels, but that method provides various sources of error. Thus, increasing use is now being made of infra-red temperature sensors carried in aircraft or satellites to obtain SST readings over relatively large areas.
Persons concerned with naval matters and, for example, fisheries and meteorologists require up-to-date forecasts of SST conditions. Charts of mean SST values are available to show expected variations of SST with latitude, but the actual temperature values are often modified considerably by ocean conditions. Thus, a system is required for accurately and efficiently determining the temperature of the air-water interface in the oceans and other large bodies of water as a means for predicting climate and weather conditions.
However, the transfer of heat between the oceans and the atmosphere is one of the most poorly understood phenomenon in climatology. Understanding the ocean-atmosphere interaction in accordance with the laws of thermodynamics is vital. It is important to understand and quantify this heat transfer in order to accurately predict weather changes.
There are a number of generally recognized methods of measuring the heat flow between the ocean and the atmosphere. The first is the bulk aerodynamics method in which measurements of wind, temperature and moisture are made a few meters above the ocean surface. Non-dimensional heat transfer coefficients are used to arrive at the heat flow from the data derived by this method. However, atmospheric instability and changes in wind velocity contribute to an expected mean error of about thirty percent for this method.
Another method involves mean dissipation which utilizes the spectral densities of wind, temperature and moisture to obtain kinetic energy, temperature and moisture variations and thus to derive the heat flow. The mean dissipation method can yield results that differ from those obtained by the bulk aerodynamics method by about thirty percent or more. The degree of error is largely dependent on whether the wind speed is increasing or decreasing.
Perhaps the most accurate method presently in use is the eddy correlation method which uses sonic anemometers to integrate vertical wind speed, temperature and moisture fluctuations. One major drawback of this method is that the recording instruments must be maintained in a vertical orientation. Any deviation from vertical results in a significant error in the measured vertical wind speed. This requirement of a vertical orientation imposes significant problems in obtaining accurate measurements in the open sea due to wave action.
Moreover, each of the foregoing methods has an additional disadvantage in that the relevant measurements are taken above the ocean surface, not at the surface itself. Thus, there is a need for a system of accurately sensing temperatures at the air-water interface of the ocean to determine heat flow between the water and the atmosphere.